Encrypts a literal string which may or may not have anything to do with RDF.

The return value is a base64-encoded string.
The base64-decoded value consists of: (1) an initialisation vector,
sixteen bytes shorter than the size of the key; (2) a 16-bit big-endian signed integer indicating the length of padding which was added to the payload of the message during encryption; (3) the payload,
encrypted using cipher-block chaining with OAEP,
with block length sixteen bytes shorter than the key size.
These three parts are concatenated together in that order.